1. Field of the Invention
The present invention relates generally to nuclear reactors, and more particularly to auxiliary support structures disposed within the lower portion of the reactor vessel for normally constraining the lower end of the core barrel in multiple-lateral directions within a horizontal plane, and for supporting the lower end of the core barrel in such a manner as to safely accommodate extraordinary vertical, radial, and/or tangential forces or loads which may be impressed upon the core barrel as a result, for example, of a natural occurrence originating outside of the reactor, or an operational or mechanical malfunction originating within the reactor, as well as for facilitating the relatively unimpeded flow of coolant throughout the lower end of the reactor vessel.
2. Description of the Prior Art
As is well known, a nuclear reactor conventionally comprises a substantially cylindrical, vertically oriented tubular reactor or pressure vessel, with the bottom of the vessel being in the form of a hollow hemispherical shell. A core vessel or barrel is disposed within the reactor or pressure vessel at an elevational level such that the bottom support plate of the core vessel or barrel is located a substantial distance above the bottom hemispherical shell wall of the reactor or pressure vessel. The reactor control rod guide tubes are of course located within the upper region of the core vessel or barrel, while the reactor core internals, comprising, for example, the fuel element assemblies, the fuel element assemblies support grid structure or framework, and the like, are of course housed within the lower region of the core vessel or barrel. Consequently, it can readily be appreciated that the core vessel or barrel embodies a considerable amount of weight.
It is therefore imperative that the core vessel or barrel be prevented from falling downwardly a substantial distance relative to the reactor or pressure vessel under, for example, seismic load or earthquake shock conditions, whereby the core vessel or barrel would tend to impact against the hemispherical bottom shell wall of the reactor or pressure vessel. Such an impact shock against the reactor or pressure vessel bottom shell wall would undoubtedly result in considerable structural damage to the reactor or pressure vessel, including the very real possibility of rupturing the reactor or pressure vessel bottom shell wall with the consequent release or leakage of the reactor core coolant out of the reactor or pressure vessel. In addition, this change in the elevational level of the core vessel or barrel relative to that of the reactor or pressure vessel would also present other safety hazards. In particular, for example, this drastic alteration in the relative disposition of the core fuel assemblies with respect to the reactor control rods would deleteriously affect the control functions of the control rods relative to the reactor core, and still further, the coolant flow throughout the reactor vessel would likewise be drastically altered.
Conventionally, therefore, the upper end of the core vessel or barrel is provided with an annular, radially outwardly extending flange, and an upper region of the reactor or pressure vessel is similarly provided with an annular, radially inwardly projecting shoulder or ledge upon which the core barrel flange may be seated. This structural system defined between the core vessel or barrel and the reactor or pressure vessel therefore constitutes the primary support means for suspendingly supporting the core vessel or barrel within the reactor or pressure vessel, however, this support system alone only serves to suspend the core vessel or barrel within the reactor or pressure vessel in a downwardly extending, cantilevered manner. If not otherwise supported, then the lower end of the core vessel or barrel will be laterally unstable within multiple lateral directions within a horizontal plane. Consequently, a further conventional practice embodied within the structural systems of nuclear reactors has been to substantially fix or support the lower end of the core vessel or barrel within the reactor or pressure vessel so as to impart lateral stability thereto. In addition, such auxiliary support means employed in connection with the lower end of the core vessel or barrel also serves as a secondary support system with respect to the aforenoted vertical loads which may be impressed upon the core vessel or barrel as a result, for example, of earthquake shock or seismic vibrational forces, core barrel of weld fractures, and the like.
One conventional type of auxiliary support system for the lower end of the reactor core barrel or vessel is disclosed within U.S. Pat. No. 3,554,868 issued to A. G. Thorp II on Jan. 12, 1971 and assigned to Westinghouse Electric Corporation, the assignee of the present application. Within this disclosed system, the central undersurface portion of the core barrel bottom support plate is supported upon the support structure which is welded to the lower central portion of the reactor or pressure vessel hemispherical shell wall. Consequently, it is apparent that while such a structural system may, in fact, suitably accomplish its lateral stability and requisite vertical load accommodation functions, it is seen that vertical static loads, as well as any vertical dynamic impact forces, will be transmitted in a substantially concentrated manner over a relatively small, localized area of the hemispherical bottom shell wall of the reactor or pressure vessel. Such load concentrations will be likely to develop high localized stressing within the central bottom portion of the reactor or pressure vessel shell wall, with the very real possibility of rupture of the same.
Another type of conventional auxiliary support system for the lower end of the reactor core barrel or vessel is of the type in which the reactor core barrel or vessel is supported by means of both a lower, central, shock-absorbing column, as well as circumferentially spaced, vertically disposed plates connected to the central, shock-absorbing column by means of upper and lower, radially extending, sets of strut members. While such a conventional support system therefore appears to resolve critical vertical load accommodation problems, resulting, for example, from seismic or earthquake vibrations or shocks, or any other similar, naturally occurring phenomenon, or mechanical or operational malfunction which would entail the imposition of extraordinary vertical load forces upon the core barrel or vessel, this system would appear to be deficient in accommodating non-radial, tangential loads impressed upon the core barrel or vessel. These loads tend to develop torque moments with respect to the circumferentially arranged core barrel support plates, and such moments generate bending and twisting stresses within such support plates and the side wall portions of the reactor or pressure vessel to which the support plates are secured.
It is to be noted further in conjunction with the last-mentioned type of conventional auxiliary support system that a solid, central, shock-absorbing support column which is disposed within the lowermost region of the reactor or pressure vessel will tend to obstruct the requisite circulation or flow of the core coolant throughout the reactor core internals and within the lowermost portion of the reactor or pressure vessel. In addition, it is also well known that as a result of the normal operation of a nuclear reactor, debris tends to be discharged into the reactor or pressure vessel, and particularly, within the core coolant. Naturally, such debris tends to accumulate within the lowermost portion of the reactor or pressure vessel under the action of gravitational forces. The various structural components of the reactor, including, for example, the shock absorber structures, are also known to be subjected to thermal expansion and contraction cycles. As often happens, the aforenoted reactor debris tends to collect within, or obstruct or clog, the various structural components of the reactor, and consequently, proper normal operation of such components, or proper operation of such components under emergency conditions, such as, for example, the shock absorber structures, is considerably hampered.
Accordingly, it is an object of the present invention to provide a new and improved auxiliary support structure for the core vessel or barrel of a nuclear reactor.
Another object of the present invention is to provide a new and improved auxiliary support structure for the core vessel or barrel of a nuclear reactor which will overcome the various aforenoted disadvantages of prior art conventional auxiliary support structures for the core vessel or barrel of a nuclear reactor.
Still another object of the present invention is to provide a new and improved auxiliary support structure for the core vessel or barrel of a nuclear reactor which will adequately provide lateral stabilization of the core vessel or barrel in multiple lateral directions within a horizontal plane under normal operating conditions of the nuclear reactor facility.
Yet another object of the present invention is to provide a new and improved auxiliary support structure for the core vessel or barrel of a nuclear reactor which will adequately provide lateral stabilization of the core vessel or barrel in multiple lateral directions within a horizontal plane, as well as vertical load accommodation, under exterior seismic or earthquake shock load conditions, or under interior operational or mechanical malfunction conditions.
Still yet another object of the present invention is to provide a new and improved auxiliary support structure for the core vessel or barrel of a nuclear reactor which will provide vertical shock absorbing capabilities for the core vessel or barrel and the reactor or pressure vessel under severe vertical dynamic load conditions.
Yet still another object of the present invention is to provide a new and improved auxiliary support structure for the core vessel or barrel of a nuclear reactor which will permit the substantial unobstructed circulation flow of coolant throughout the reactor internals, and particularly within the lower region of the reactor or pressure vessel.
A further object of the present invention is to provide a plurality of new and improved auxiliary support structures equiangularly located about the inner periphery of the reactor or pressure vessel for supporting the core vessel or barrel within the reactor or pressure vessel in such a manner that all loads impressed upon the core vessel or barrel, and transmitted to the auxiliary support structures, are optimally distributed to the sidewalls of the reactor or pressure vessel, and not exclusively concentrated over a relatively small, single, localized wall portion of the reactor or pressure vessel.
A still further object of the present invention is to provide a new and improved auxiliary support structure for the core vessel or barrel of a nuclear reactor which can provide vertical shock-absorbing capabilities to the core vessel or barrel and adequately support the same under severe dynamic vertical loading so as to prevent dynamic impact forces from being transmitted to the reactor or pressure vessel, and thereby preventing rupture of the reactor or pressure vessel.
A yet further object of the present invention is to provide a new and improved auxiliary support structure for the core vessel or barrel of a nuclear reactor which can adequately limit the vertical drop of the core vessel or barrel under severe vertical load conditions so as to maintain substantially the same elevational level of the core vessel or barrel for safe interaction of the core fuel assemblies with the reactor control rods, as well as with the circulating flow of coolant.
An additional object of the present invention is to provide a new and improved auxiliary support structure for the core vessel or barrel of a nuclear reactor wherein such structures facilitate the elimination of the need for the conventional, massive auxiliary support structures which required an inordinate amount of welding to be performed between such support structures and the reactor or pressure vessel sidewalls.